CN108883281B

CN108883281B - Atrial tracking in an intracardiac ventricular pacemaker

Info

Publication number: CN108883281B
Application number: CN201780019976.5A
Authority: CN
Inventors: S·戈什; S·克莱克; T·J·谢尔登
Original assignee: Medtronic Inc
Current assignee: Medtronic Inc
Priority date: 2016-03-22
Filing date: 2017-03-02
Publication date: 2022-06-24
Anticipated expiration: 2037-03-02
Also published as: US10080900B2; CN108883281A; US20170274213A1; EP3432976B1; WO2017165096A1; EP3432976A1

Abstract

The intracardiac ventricular pacemaker is configured to detect atrial mechanical events from motion sensor signals received by atrial event detector circuitry of the pacemaker. The motion sensor signal is responsive to the motion of blood flowing in the heart chamber. At expiration of a pacing interval set by the pacing timing circuit, a pacing pulse is scheduled in response to detecting the atrial mechanical event. Atrial synchronous ventricular pacing pulses are delivered at the expiration of the pacing interval.

Description

Atrial tracking in an intracardiac ventricular pacemaker

Technical Field

The present disclosure relates to intracardiac ventricular pacemakers and associated methods for detecting atrial events and controlling atrial-synchronized ventricular pacing delivered by the pacemakers using motion sensors.

Background

Implantable intracardiac pacemakers are often placed in a subcutaneous pocket and coupled to one or more transvenous medical electrical leads carrying pacing and sensing electrodes positioned in the heart. A subcutaneously implanted cardiac pacemaker may be a single chamber pacemaker coupled to one transvenous medical lead for positioning the electrodes in one heart chamber (either the atrium or ventricle), or a dual chamber pacemaker coupled to two intracardiac leads for positioning the electrodes in both the atrial and ventricular chambers. Multi-chamber pacemakers are also available that may be coupled to, for example, three leads for positioning electrodes in one atrial chamber and both the right and left ventricles for pacing and sensing.

Intracardiac pacemakers have recently been introduced that are implantable within a ventricular chamber of a patient's heart for delivering ventricular pacing pulses. Such a pacemaker may sense an R-wave signal that accompanies intrinsic ventricular depolarization and deliver a ventricular pacing pulse without sensing an R-wave. While single chamber ventricular sensing and pacing by an intracardiac ventricular pacemaker may be sufficient to address some patient conditions, other conditions may require atrial and ventricular (dual chamber) sensing for providing atrial-synchronized ventricular pacing in order to maintain a regular heart rhythm.

Drawings

Fig. 1 is a conceptual diagram illustrating an intracardiac pacing system that may be used to sense cardiac electrical signals, cardiac accelerometer signals, and provide pacing therapy to a patient's heart.

Fig. 2A is a conceptual diagram of the intracardiac pacemaker shown in fig. 1.

Fig. 2B is a conceptual diagram of an alternative example of the pacemaker of fig. 1.

FIG. 2C is an end view of one example of a flow-perturbing fixture.

Fig. 2D is a side view of another example of a flow-perturbing fixture.

Fig. 2E is a side view of another example of a flow-perturbing fixture.

Fig. 2F is a top view of another example of a flow-perturbing fixture.

Fig. 3 is a schematic diagram of an example configuration of the pacemaker shown in fig. 1.

Fig. 4 is a schematic diagram of an atrial event detector circuit of an intracardiac pacemaker according to one example.

Fig. 5 is a flow diagram of a method for delivering atrial-synchronized ventricular pacing according to one example.

Fig. 6 is a diagram of an accelerometer signal, an EGM signal, and timing diagrams representing operations performed by a pacemaker according to the techniques disclosed herein.

Detailed Description

Fig. 1 is a conceptual diagram illustrating an intracardiac pacing system 10 that may be used to sense cardiac electrical signals and motion signals caused by flowing blood and provide pacing therapy to a patient's heart 8. IMD system 10 includes a Right Ventricular (RV) intracardiac pacemaker 14 and may optionally include a Right Atrial (RA) intracardiac pacemaker 12 in some examples.

Pacemakers

12 and 14 are transcatheter intracardiac pacemakers that may be adapted for implantation entirely within a heart chamber of heart 8, e.g., entirely within the RV, entirely within the Left Ventricle (LV), entirely within the RA, or entirely within the Left Atrium (LA). In the example of fig. 1, pacemaker 12 is positioned along an endocardial wall of the RA (e.g., along a lateral wall of the RA or a septum of the RA). Pacemaker 14 is positioned along the endocardial wall of the RV (e.g., near the RV apex). However, the techniques disclosed herein are not limited to the pacemaker locations shown in the example of fig. 1, and other locations and relative positions to each other are possible. In some examples, a ventricular intracardiac pacemaker 14 is positioned in the LV for delivering atrial-synchronized ventricular pacing using the techniques disclosed herein.

Pacemakers

12 and 14 are reduced in size compared to subcutaneously implanted pacemakers, and may be generally cylindrical in shape to enable transvenous implantation via a delivery catheter. In other examples,

pacemakers

12 and 14 may be positioned at any other location within heart 8. For example, pacemaker 12 may be positioned outside or within the right or left atrium to provide respective right or left atrial pacing. Pacemaker 14 may be positioned within either the right or left ventricle to provide respective right or left ventricular pacing and for sensing blood flow by a motion sensor within the ventricular cavity.

Pacemakers

12 and 14 are each capable of producing electrical stimulation pulses, e.g., pacing pulses, that are delivered to heart 8 via one or more electrodes on an external housing of the pacemaker. RA pacemaker 12 is configured to sense cardiac electrical signals from within the RA, which may be used to generate an RA intracardiac Electrogram (EGM) signal. The RA cardiac electrical signals may be sensed using housing-based electrodes that are also used to deliver RA pacing pulses. RV pacemaker 14 is configured to deliver RV pacing pulses using the housing-based electrodes and sense RV cardiac electrical signals for producing RV EGM signals.

In some examples, a patient may only need RV pacemaker 14 for delivering ventricular pacing. In other examples, RA pacemaker 12 may be required for delivering atrial pacing depending on individual patient needs. RV pacemaker 14 is configured to control the delivery of ventricular pacing pulses to the RV in a manner that facilitates synchronization between RA and RV activation (e.g., by maintaining a target Atrioventricular (AV) interval between atrial events and ventricular pacing pulses). That is, RV pacemaker 14 controls RV pacing pulse delivery to maintain a desired AV interval between atrial activation (intrinsic or pacing-evoked) corresponding to atrial contraction and ventricular pacing pulses delivered to cause ventricular depolarization. In accordance with the techniques described herein, atrial activation is detected by RV pacemaker 14 using a motion sensor signal that is responsive to blood flow into the RV due to atrial activation. For example, the acceleration of blood flow into the RV caused by atrial activation through tricuspid valve 16 between the RA and RV (sometimes referred to as "atrial kick") is detected by RV pacemaker 14 from signals generated by motion sensors (e.g., an accelerometer included in RV pacemaker 14).

The P-wave of the near-field RV electrocardiographic signal that accompanies atrial depolarization is a relatively low amplitude signal (e.g., compared to the R-wave) and, therefore, may be difficult to reliably detect from the electrocardiographic signal acquired by RV pacemaker 14. Therefore, atrial-synchronized ventricular pacing by RV pacemaker 14 may be unreliable when based solely on the cardiac electrical signals received by RV pacemaker 14. In accordance with the techniques disclosed herein, RV pacemaker 14 includes a motion sensor (such as an accelerometer) and is configured to detect atrial events corresponding to atrial mechanical activation or atrial contraction using signals from the motion sensor. The ventricular pacing pulses are synchronized to atrial events detected from the accelerometer signal by a programmable AV interval.

The target AV interval may be a programmed value selected by a clinician and is the time interval from the detection of an atrial event until the delivery of a ventricular pacing pulse. In some instances, the target AV interval may begin from the time an atrial event is detected based on the motion sensor signal, or from an identified fiducial point of the motion sensor signal. The target AV interval may be identified as being hemodynamically optimal for a given patient based on clinical testing or assessment of the patient or based on clinical data from a population of patients. The target AV interval may be determined to be optimal based on the relative timing of the electrical and blood flow-related events identified from the cardiac electrical signals received by RV pacemaker 14 and the motion sensor signals received by RV pacemaker 14.

Pacemakers

12 and 14 may each be capable of bidirectional wireless communication with external device 20 for programming AV intervals as well as other blood flow motion sensing parameters and pacing control parameters. Aspects of external device 20 may generally correspond to the external programming/monitoring unit disclosed in U.S. patent No.5,507,782(Kieval et al). External device 20 is often referred to as a "programmer" because it is typically used by a physician, technician, nurse, clinician, or other qualified user for programming operating parameters in

pacemakers

12 and 14. The external device 20 may be located in a clinic, hospital, or other medical facility. The external device 20 may alternatively be embodied as a home monitor or handheld device that may be used in a medical facility, in the home of a patient, or at another location. Operating parameters, including sensing and therapy delivery control parameters, may be programmed into

pacemakers

12 and 14 using external device 20.

External device 20 is configured for interfacing with implantable telemetry circuitry included in RV pacemaker 14 and RA pacemaker 12 (when present)The line communicates bi-directionally. External device 20 establishes a wireless Radio Frequency (RF) communication link 22 with RA pacemaker 12 and a wireless RF communication link 24 with RV pacemaker 14 using a communication protocol that appropriately addresses the

target pacemaker

12 or 14. May use a signal such as

A Wi-Fi, Medical Implant Communication Service (MICS), or other RF link such as communication bandwidth to establish

communication links

22 and 24. In some examples, external device 20 may include a programming head that is placed adjacent to

pacemaker

12 or 14 to establish and maintain a communication link, and in other examples, external device 20 and

pacemaker

12 or 14 may be configured to communicate using a distance telemetry algorithm and circuitry that does not require the use of a programming head and does not require user intervention to maintain a communication link. An example RF telemetry communication system that may be implemented in the system 10 is generally disclosed in U.S. patent No.5,683,432(Goedeke et al).

External device 20, during an interrogation session, may display data and information related to pacemaker function to a user for viewing pacemaker operation and programmed parameters as well as EGM signals acquired by pacemaker 14 or pacemaker 12, motion sensor signals acquired by pacemaker 14, or other physiological data acquired by pacemakers 12 and/or 14 and retrieved from pacemakers 12 and/or 14.

It is contemplated that the external device 20 may be wired or wirelessly connected to a communication network via telemetry circuitry including a transceiver and antenna or via a hardwired communication line for communicating data to a remote database or computer to allow remote management of the patient. A remote patient management system including a remote patient database may be configured to utilize the presently disclosed techniques to enable a clinician to view EGM, motion sensor and marker channel data and authorize programming of sensing and therapy control parameters in RV pacemaker 14 after viewing a visual representation of the EGM, accelerometer and marker channel data.

Pacemaker 12 and pacemaker 14 may or may not be configured to communicate directly with each other. When

pacemakers

12 and 14 are configured to communicate with each other, the communication may be minimized in order to conserve battery life of

intracardiac pacemakers

12 and 14. As such, communication may not occur on a beat-by-beat (beat-bed) basis between RA pacemaker 12 and RV pacemaker 14 for communication when another pacemaker is sensing cardiac events or when it is delivering pacing pulses. As disclosed herein, RV pacemaker 14 is configured to detect atrial events from motion sensor signals as typically frequent on a beat-by-beat basis, without requiring communication signals from RA pacemaker 12 to provide atrial event detection for controlling atrial-synchronized ventricular pacing.

Fig. 2A is a conceptual diagram of intracardiac RV pacemaker 14 shown in fig. 1. RV pacemaker 14 includes

electrodes

162 and 164 spaced along housing 150 of pacemaker 14 for sensing cardiac electrical signals and delivering pacing pulses. Electrode 164 is shown as a tip electrode extending from distal end 102 of pacemaker 14, and electrode 162 is shown as a ring electrode along a middle portion of housing 150 (e.g., adjacent proximal end 104). The distal end 102 is referred to as "distal" because the distal end 102 is expected to be the leading end (leading end) when the pacemaker 14 is advanced through a delivery tool, such as a catheter, and placed against a target pacing site.

Electrodes

162 and 164 form an anode and cathode pair for bipolar cardiac pacing and sensing. In alternative embodiments, pacemaker 14 may include two or more ring electrodes, two tip electrodes, and/or other types of electrodes exposed along pacemaker housing 150 for delivering electrical stimulation to heart 8 and sensing cardiac electrical signals.

Electrodes

162 and 164 may be, but are not limited to, titanium, platinum, iridium, or alloys thereof, and may include low polarization coatings such as titanium nitride, iridium oxide, ruthenium oxide, platinum black, and the like.

Electrodes

162 and 164 may be positioned at locations along pacemaker 14 other than the locations shown.

The housing 150 is formed of a biocompatible material, such as stainless steel or a titanium alloy. In some examples, the housing 150 may include an insulating coating. Examples of insulating coatings include parylene, polyurethane, PEEK, or polyimide, among others. The entirety of housing 150 may be insulated, but only

electrodes

162 and 164 are uninsulated. Electrode 164 may serve as a cathode electrode and be coupled to internal circuitry enclosed by the housing, e.g., a pacing pulse generator and cardiac electrical signal sensing circuitry, via an electrical feedthrough across housing 150. The electrode 162 may be formed as a conductive portion of the housing 150, as a ring electrode, that is electrically isolated from other portions of the housing 150, as generally shown in fig. 2A. In other examples, rather than providing a partial ring electrode such as anode electrode 162, the entire perimeter of housing 150 may be used as an electrode that is electrically isolated from tip electrode 164. Electrode 162, formed along a conductive portion of housing 150, serves as a return anode during pacing and sensing.

Housing 150 includes control electronics subassembly 152 that houses electronics for sensing cardiac signals, generating pacing pulses, and controlling therapy delivery, as well as other functions of pacemaker 14 as described below in connection with fig. 3. In some examples, the motion sensor may be implemented as an accelerometer enclosed within the housing 150. The accelerometer provides a signal to a processor included in the control electronics subassembly 152 for detecting atrial events.

The housing 150 further includes a battery subassembly 160, the battery subassembly 160 providing power to the control electronics subassembly 152. The battery subassembly 160 may include the features of the batteries disclosed in commonly assigned U.S. patent No.8,433,409(Johnson et al) and U.S. patent No.8,541,131(Lund et al).

Pacemaker 14 may include a set of fixation tines 166 to secure pacemaker 14 to patient tissue, for example, by actively engaging the ventricular endocardium and/or interacting with the ventricular trabecula. The fixation tines 166 are configured to anchor the pacemaker 14 to position the electrodes 164 operably adjacent target tissue for delivery of therapeutic electrical stimulation pulses. Many types of active and/or passive fixation members may be employed to anchor or stabilize pacemaker 14 in the implanted position. Pacemaker 14 may include a set of fixation tines as disclosed in commonly assigned, pre-authorized publication US2012/0172892(Grubac et al).

Pacemaker 14 may optionally include a delivery tool interface 158. Delivery tool interface 158 may be located at proximal end 104 of pacemaker 14 and configured to connect to a delivery device, such as a catheter, for positioning pacemaker 14 at an implantation location, e.g., within a heart chamber, during an implantation procedure.

Pacemaker housing 15, including delivery tool interface 158, is generally smooth, wirelined and radially symmetric to cause minimal flow disturbances or turbulence when implanted in a ventricle. For example, the symmetry and low profile of the delivery tool interface 158 is designed to minimize flow disturbances. However, in some examples disclosed herein, pacemaker 14 includes structures or fixtures that intentionally increase vibration of housing 150 when subjected to flowing blood, particularly when subjected to blood acceleration and flow into the ventricle during atrial systole. Such "flow perturbation structures" or anchors may introduce radial asymmetry into pacemaker 14 and/or protrude from housing 150, with length and flexibility (or stiffness) that vibrates when subjected to flowing blood, particularly blood flowing through tricuspid valve 16 into the ventricle due to atrial contraction. Vibrations caused by the flow perturbating structure or fixture are transmitted to the motion sensor within the housing 150. Structures or fixtures designed to increase the vibration of housing 150 when subjected to blood flowing into the ventricles due to atrial contraction may enhance atrial event detection based on the motion sensor signals.

In the example shown in fig. 2A, the housing 150 may include a transverse flow-perturbation groove 168, the flow-perturbation groove 168 extending longitudinally along the perimeter of the housing 150. The flow-perturbing grooves 168 may be coated to prevent blood from condensing along the grooves. The groove 168 may extend between the proximal and

distal ends

104, 102 for at least a portion of the length of the housing 150 and may be at any angle relative to the longitudinal axis. In fig. 2A, the groove 168 extends from the proximal end 104 along the battery subassembly 160 and terminates before the control electronics subassembly 152. The flow-perturbating trough 168 may extend along the length of the housing 150 for a longer or shorter distance than shown. In other examples, housing 150 may include structures or fixtures provided as coatings or protrusions (such as lateral ridges or flanges) that provide pacemaker housing 150 with a degree of radial asymmetry such that blood flowing through housing 150 causes vibrations of housing 150 that cause motion sensors within housing 150 to generate signals related to those vibrations. As the atrium contracts and the blood flow rate through the tricuspid valve 16 into the right ventricle (or through the mitral valve into the left ventricle) increases, the increase in vibration of the housing 150 causes a related increase in the frequency of the spikes, the amplitude of the spikes, and/or the slope of the spikes, or other changes in the motion sensor signal caused by vibrations that may be detected by atrial event detector circuitry (described in connection with fig. 4) included in the control electronics subassembly 152.

Fig. 2B is a conceptual diagram of an alternative example of RV pacemaker 14. RV pacemaker 14 includes housing 150, control electronics subassembly 152, battery subassembly 160, fixation member 166, and electrode 164 along distal end 102, as described above in connection with fig. 2A. Pacemaker 14 is shown to include a flow-disrupting fixation member 165 extending from proximal end 104. The mount 165 may be a flexible mount configured to vibrate or vibrate when exposed to flowing blood. The fixture 165 may be configured to vibrate in the flowing blood. During atrial contraction, the increased vibration of the fixation member 165 during active ventricular filling phases may be caused by blood flowing into the ventricle via the tricuspid valve 16. Steady state blood flow may cause the mount 165 to vibrate at a resonant frequency. The acceleration or change in direction of blood flowing along the ventricular inflow tract may change the vibrations. For example, vibrations may increase in amplitude, slope, frequency, direction, and the like. The motion sensors included in pacemaker 14 generate signals that include these vibrations, which are analyzed to detect atrial mechanical contractions.

Fixation 165 is shown as a tapered fixation having a generally conical shape extending distally from proximal end 104 of pacemaker 14. The narrower proximal end 172 may have greater flexibility than the wider distal end 170 and experience greater vibration in the flowing blood, which may increase in amplitude as blood flow through the tricuspid valve increases. The fixation member 165 may be positioned relative to the inflow tract of the right ventricle such that the vibrations of the fixation member 165 increase during atrial contraction.

In some examples, fixation member 165 may be radially symmetric and aligned with the central axis of pacemaker 14, but have a length extending away from housing 150, flexibility of at least proximal end 172, and orientation relative to the inflow tract, which results in vibrations of increased amplitude and transmitted to fixation member 165 of housing 150 during atrial contractions. The relatively larger diameter, stiffer distal end 170 coupled to housing proximal end 104 transmits the more flexible proximal vibrations to housing 150 and the motion sensor included in pacemaker 14. The fixture 165 may be provided with a stiffness and length that produces small amplitude vibrations (e.g., without limitation intended, no more than 2mm, no more than 1mm, or no more than 0.5mm of excursion) when subjected to blood flowing along the right ventricular inflow tract, which may have a flow velocity of about 0.3 to 1.7 meters/second.

In some examples, the mount 165 is radially asymmetric to promote vibration of the mount in flowing blood. For example, the fixture 165 may have a relatively flat geometry, as shown in an end view of the fixture 165 in fig. 2C. The distal end 170 coupled to the pacemaker housing 150 may be provided with a width 174 greater than its height 176. The securing member 165 may taper from its distal end 170 to its proximal end 172. The relatively flat geometry may increase the bending or vibration of the mount 165 in a particular direction (e.g., generally up and down movement corresponding to the smaller dimension 176 and lateral movement corresponding to the larger dimension 174 in the orientation shown in fig. 2C). This directionality of the increase in motion or vibration of the fixation member 165 caused by accelerated blood flowing through the tricuspid valve 16 into the RV may correspond to the axis of an accelerometer (or other motion sensor) included in RV pacemaker 14, such that accelerometer signals along one axis of the accelerometer are sensitive to up and down motion along the vertical axis of member 165 in the orientation shown in fig. 2C. In other words, the accelerometer axis may be aligned with the short axis of the mount 165, which is expected to have the greatest motion and sensitivity to blood flow acceleration through the tricuspid valve 16 during atrial activation.

The fixture 165 can have a first stiffness along a first fixture axis (e.g., corresponding to the width 174) and a second stiffness along a second fixture axis (e.g., corresponding to the thickness 176) that can be orthogonal to the first fixture axis that is less than the first stiffness. The axis of lower stiffness may be aligned with the axis of motion of the accelerometer such that increased vibration of the mount along the axis of lower stiffness has a directionality aligned with the axis of motion of the accelerometer. When the accelerometer is enclosed by the housing 15 or mounted to the housing 15, increased vibration of the mount 165 is transmitted to the accelerometer via the distal mount end 170 coupled to the housing 15 to improve detection of atrial mechanical events from the accelerometer signal.

In other examples, the fixture 165 may have a substantially equal thickness 176 and width 174 at the distal end 170, and may taper toward the proximal end 172, or have a continuous dimension along most or all of its length between the distal end 170 and the proximal end 172. For example, as shown in fig. 2D, flow-perturbing fixture 165 'has a generally cylindrical shape from its proximal end 170' to its distal free end 172', the proximal end 170' being coupled to pacemaker housing proximal end 104. In the example of fig. 2D, member 165 'has a non-tapered continuous diameter that terminates in a blunt or rounded distal end 172'.

The fixture 165 (and other examples of flow-perturbing fixtures presented herein) may be a molded component formed from silicone, polyurethane, epoxy, or other biocompatible polymers. When formed of a polymer, the securing member 165 may be fixedly coupled to the housing proximal end 104 via an epoxy, silicone adhesive, or other medical grade adhesive. In some examples, the housing proximal end 104 may define an opening or window 105 through which the fastener 165 protrudes. For example, the retainer 165 may have an inner ring, lip, or flange 175 that extends within the housing 150 against the inner surface of the housing proximal end 104 when the outer portion of the retainer 165 shown in fig. 2B is advanced through the opening 105 in the housing proximal end 104 during manufacturing assembly. The window 105 defined by the housing proximal end 104 may then be sealed with a medical adhesive to maintain the hermeticity of the housing 150.

In other examples, the fixture 165 may be a wire, ribbon, molded or stamped component formed from a metal (such as stainless steel or a titanium alloy) and may be welded to the housing proximal end 104 or within the window 105 defined by the housing proximal end 104. The fixture 165 may be an integral part of a homogeneous material, or comprise a core of polymer or metal that is overmolded or coated with a polymer layer to provide the desired shape and/or stiffness. In some examples, the stiffness of the mount 165 is substantially constant from the proximal end 170 to the distal end 172, and in other examples, the mount 165 has a variable stiffness, e.g., a lower stiffness near the proximal end 172 and a greater stiffness near the distal end 170 to provide greater bending and vibration at the proximal end 172 that can be transmitted to the accelerometer via the inner core without imparting increased vibration or motion on the housing 150. The fixture 165 may simply be a mechanical part extending from the housing 150 without any electrodes, electrical conductors, or other electrical elements or functions. Mount 165 transmits mechanical vibrations to housing 150, or directly to a motion sensor or accelerometer, via a relatively rigid connection between mount 165 and housing 150, and thus may have a stiffness, including a stiffness at the point of connection to housing 150 that is greater than the relatively flexible medical electrical lead body that carries the sensing and pacing electrodes and is sometimes coupled to a pacemaker.

In some cases, the anchor 165 may be adjustable when implanted at a particular intraventricular location such that it may be oriented in a direction that increases the vibrations caused by the flow of blood from the atrium into the ventricle. For example, the mount 165 may be rotatable such that it may be rotated up to 90 degrees (or more) relative to the motion sensor within the housing 150 and relative to the direction of blood flow along the right (or left) ventricular inflow tract to change the primary axis of vibration of the mount 165.

Fig. 2E is a side view and fig. 2F is a top view of another example of a flow-disrupting fixture 165 ". In this example, the flow-perturbing fixture 165 "comprises a distal rod 184 extending from the housing proximal end 104 and a proximal paddle 180 carried by the rod 184. When the rod 184 extends through the open window 105 defined by the housing proximal end 104, the rod 184 may extend from the inner flange 185, the inner flange 185 being positioned against the inner surface of the housing proximal end 104. In other examples, the rod 184 is fixedly coupled directly to the outer surface of the housing proximal end 104.

Proximal paddle 180 may be a flat elliptical, circular, rectangular, or other shaped paddle, or may be generally spherical, conical, or pyramidal in shape. The proximal paddle 180 may be symmetrical or asymmetrical about the shaft 184 to enhance vibration of the fixture 165 "in response to acceleration of blood flow through the tricuspid valve 16. In the example shown, paddle 180 includes a recess or depression 182 along one side of paddle 180. The recess 182 may increase the oscillating motion of the fixture 165 "in the up and down direction corresponding to the size of the blade thickness 187, rather than the side-to-side motion corresponding to the size of the blade width 183. In this example, the width 183 is greater than the thickness 187. Lateral movement in a direction parallel to the blade width 183 and up and down movement parallel to the blade thickness 187 may be enhanced by the relatively high flexibility (lower stiffness) of the bar 184. The sensitivity of motion in both the lateral and up-down directions may reduce orientation dependence of detecting atrial events based on detecting blood flow acceleration due to atrial events from accelerometer signals (or other motion signals). However, specificity for blood flow acceleration due to atrial activation rather than ventricular activation or other causes may be increased by designing the fixation member 165 "to have a stiffness and shape that results in greater vibration in response to increased blood flow through the tricuspid valve 16, rather than from/in other directions (e.g., from the apex of the RV, along the RV outflow tract, or along the sidewall of the RV).

Fig. 3 is a schematic diagram of an example configuration of pacemaker 14 shown in fig. 1. Pacemaker 14 includes a pulse generator 202, a sensing circuit 204, a control circuit 206, a memory 210, a telemetry circuit 208, a motion sensor 212, and a power source 214. Motion sensor 212 is implemented as an accelerometer in the examples described below in connection with fig. 4-6 and is also referred to herein as "accelerometer 212". However, motion sensor 212 is not limited to being an accelerometer, and other motion sensors may be successfully utilized in pacemaker 14 to detect atrial events according to the techniques described herein. Examples of motion sensors include piezoelectric sensors and micro-electromechanical system (MEMS) devices. Motion sensor 212 may be a multi-axis sensor (e.g., a two-dimensional or three-dimensional sensor) with each axis providing a signal that may be analyzed separately or in combination for detecting atrial mechanical contractions. The various circuits represented in fig. 3 may be combined on one or more integrated circuit boards, including an Application Specific Integrated Circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that execute one or more software or firmware programs, a combinational logic circuit, a state machine, or other suitable components that provide the described functionality.

The sensing circuit 204 is configured to receive cardiac electrical signals via the

electrodes

162 and 164 through a pre-filter and amplifier circuit 220. The pre-filter and amplifier circuit may include a high pass filter that removes DC offset, such as a 2.5 to 5Hz high pass filter, or a wideband filter having a 2.5Hz to 100Hz passband to remove DC offset and high frequency noise. The pre-filter and amplifier circuit 220 may further include an amplifier to amplify the "raw" cardiac signal that is passed to the rectifier and amplifier circuit 222. The rectifier and amplifier circuit 222 may include a rectifier, a band pass filter, and an amplifier for passing the cardiac signal to an R-wave detector 224 and an analog-to-digital converter (ADC) 226. ADC 226 may deliver a multi-bit digital Electrogram (EGM) signal to control circuit 206 for use by atrial event detector circuit 240 to identify ventricular electrical events (e.g., R-waves or T-waves) and blanking (blank) or ignore ventricular event intervals from an accelerometer signal used to detect atrial events.

The R-wave detector 224 may include a sense amplifier or other detection circuitry that compares the incoming rectified electrocardiographic signal to an R-wave detection threshold, which may be an automatically adjusted threshold. When the input signal crosses the R-wave detection threshold, the R-wave detector 224 generates an R-wave sense event signal (R-sense), which is passed to the control circuit 206. In other examples, the R-wave detector 224 may receive the digital output of the ADC 226 for detecting R-waves through a comparator, morphological signal analysis of the digital EGM signal, or other R-wave detection techniques.

Control circuit 206 includes atrial event detector circuit 240, pacing timing circuit 242, and processor 244. Atrial event detector circuit 240 is configured to detect atrial events from signals received from motion sensor 212. Motion sensor 212 may be a piezoelectric crystal or a micro-electromechanical system (MEMS) device that, for example, when subjected to flowing blood, generates an electrical signal related to the motion or vibration of sensor 212 (and pacemaker 14). Motion sensor 212 may be a one-dimensional single-axis accelerometer, a two-dimensional or three-dimensional multi-axis accelerometer. The motion sensor 212 may be configured with an accelerometer axis aligned with the axis of motion of the flow-perturbing fixture 165, which is expected to be aligned with the maximum displacement of the fixture 165 when subjected to acceleration of blood flow through the tricuspid valve 16. One example of an accelerometer for an implantable medical device is generally disclosed in U.S. patent No.5,885,471 (Ruben et al). Implantable medical devices including piezoelectric accelerometers for detecting patient motion are disclosed, for example, in U.S. patent No.4,485,813(Anderson et al) and U.S. patent No.5,052,388(Sivula et al). Three-dimensional accelerometers for sensing patient activity and posture are generally described in U.S. Pat. No.5,593,431(Sheldon) and U.S. Pat. No.6,044,297 (Sheldon). Other accelerometer designs may be used to generate electrical signals related to motion imparted on pacemaker 14 or at least on motion sensor 212, which may be blood flow induced motion or vibration transmitted to mount 165 of motion sensor 212 via mount 165.

Atrial event detector circuit 240 may receive the R-wave sensed event signal and/or the digital EGM signal from sensing circuit 204 for setting a ventricular blanking period, which is applied to the motion sensor signal, to avoid erroneously detecting a ventricular event as an atrial event, as described in more detail below in conjunction with fig. 4 and 5. Briefly, atrial event detector circuit 240 may receive raw signals from one or more axes of motion sensor 212, apply a ventricular blanking period based on the timing of ventricular electrical events determined from signals from sensing circuit 204, and determine whether motion sensor signals outside of the ventricular blanking period satisfy atrial mechanical event detection criteria. The motion sensor signal during the ventricular blanking period may be ignored for purposes of detecting atrial events, but may be used for other purposes, such as detecting ventricular mechanical events. In other examples, rather than setting the ventricular blanking period, the atrial event detection window may be set based on the timing of the ventricular electrical or mechanical event, and the atrial event detector circuit 240 may determine whether the atrial mechanical event detection criteria are met within the atrial event detection window.

The term "blanking" as used herein in reference to a ventricular blanking period applied to a motion sensor signal is not necessarily an absolute blanking period in which an electrical signal generated by the motion sensor is blanked. In contrast, ventricular blanking periods generally refer to time intervals applied to the motion sensor signal to distinguish between atrial mechanical events and ventricular mechanical events. The detection of atrial mechanical events does not occur during the ventricular blanking period. However, motion sensor signals received during the ventricular blanking period may be processed or analyzed for other purposes, such as detecting ventricular mechanical events associated with electrical depolarization and repolarization of the ventricles. Detection of these ventricular mechanical events may be used, for example, to set a ventricular blanking period that applies to limit atrial event detection that occurs outside of the ventricular blanking period, to control an atrial event detection window applied to the motion sensor signal, and/or to control a refractory period applied to the cardiac electrical signal. The circuitry included in atrial event detector circuit 240 and techniques for detecting atrial mechanical events are described below in conjunction with FIG. 4.

Atrial event detector circuit 240 passes the atrial event detection signals to processor 244 and/or pace timing circuit 242. Pace timing circuit 242 (or processor 244) may additionally receive R-wave sensed event signals from R-wave detector 224 for controlling the timing of pacing pulses delivered by pulse generator 202. The processor 244 may include one or more clocks for generating clock signals used by the pace timing circuit 242 to count down the AV pacing interval that begins upon receipt of an atrial event detection signal from the atrial event detector circuit 240. The pacing timing circuit 242 may include one or more pacing escape interval timers or counters for counting down AV pacing intervals, which may be programmable intervals stored in memory 210 and retrieved by processor 244 for use in setting the AV pacing intervals used by the pacing timing circuit 242. Processor 244 may retrieve other programmable pacing control parameters (such as pacing pulse amplitude and pacing pulse width) that are passed to pulse generator 202 for use in controlling pacing pulse delivery. In addition to providing control signals to pacing timing circuit and pulse generator 202 for controlling pacing pulse delivery, processor 244 may provide sensing control signals to sensing circuit 204, such as R-wave sensing thresholds, sensitivities, various blanking and refractory period intervals applied to the cardiac signal, and atrial event detection control signals to atrial event detector circuit 240, such as ventricular blanking period durations, atrial detection thresholds applied to motion signals, and other atrial event detection criteria applied by circuitry included in atrial event detector circuit 240.

The functionality attributed herein to pacemaker 14 may be embodied as one or more processors, controllers, hardware, firmware, software, or any combination thereof. Depiction of different features as specific circuits is intended to highlight different functional aspects and does not necessarily imply that such functions must be realized by separate hardware, firmware, or software components or by any particular circuit architecture. Rather, the functionality associated with one or more circuits described herein may be performed by separate hardware, firmware, or software components, or integrated within a common hardware, firmware, or software component. For example, atrial mechanical event detection and ventricular pacing control operations performed by pacemaker 14 may be implemented in control circuitry 206, with control circuitry 206 executing instructions stored in memory 210 and relying on inputs from sensing circuitry 204 and motion sensor 212.

The operation of circuitry included in pacemaker 14 as disclosed herein should not be construed as reflecting specific forms of hardware, firmware and software necessary to practice the described techniques. It is believed that the particular form of software, hardware, and/or firmware will be determined primarily by the particular system architecture employed in pacemaker 14 and by the particular sensing circuitry and therapy delivery circuitry employed by pacemaker 14. It is within the ability of those skilled in the art, given the disclosure herein, to provide software, hardware, and/or firmware for accomplishing the described functions in the context of any modern pacemaker.

Pulse generator 202 generates an electrical pacing pulse that is delivered to the RV of the patient's heart via

electrodes

162 and 164. The pulse generator 202 may include a charging circuit 230, a switching circuit 232, and an output circuit 234. Charging circuit 230 may include a holding capacitor that may be charged to the pacing pulse amplitude by a multiple of several battery voltage signals of power supply 214 under the control of a voltage regulator. The pacing pulse amplitude may be set based on a control signal from the control circuit 206. The switching circuit 232 may control when the holding capacitor of the charging circuit 230 is coupled to the output circuit 234 for delivering pacing pulses. For example, the switching circuit 232 may include a switch that is activated by a timing signal received from the pacing timing circuit 242 at the expiration of the AV pacing interval and remains closed for a programmed pacing pulse duration to enable the holding capacitor of the charging circuit 230 to discharge. The hold capacitor previously charged to the pacing pulse voltage amplitude is discharged across

electrodes

162 and 164 through the output capacitor of output circuit 234 for the programmed pacing pulse duration. Examples of pacing circuits generally disclosed in U.S. patent No.5,507,782(Kieval et al) and in commonly assigned U.S. patent No.8,532,785(Crutchfield et al) may be implemented in pacemaker 14 for charging a pacing capacitor to a predetermined pacing pulse amplitude and delivering pacing pulses under the control of control circuit 206.

Memory 210 may include computer readable instructions that, when executed by control circuitry 206, cause control circuitry 206 to perform various functions attributed throughout this disclosure to pacemaker 14. The computer readable instructions may be encoded within memory 210. Memory 210 may include any non-transitory computer-readable storage medium including any volatile, non-volatile, magnetic, optical, or electrical medium, such as Random Access Memory (RAM), read-only memory (ROM), non-volatile RAM (nvram), electrically erasable programmable ROM (eeprom), flash memory, or other digital media, with the sole exception being transitory propagating signals. In accordance with the techniques disclosed herein, memory 210 may store timing intervals and other data used by control circuitry 206 to control the delivery of pacing pulses by pulse generator 202 (e.g., by detection of atrial mechanical events by atrial event monitor circuitry 240 and setting a pacing escape interval timer included in pacing timing circuitry 242).

Power source 214 provides power to each of the other circuits and components of pacemaker 14 as needed. Control circuitry 206 may perform power control operations to control when power is supplied to various circuits or components to perform various pacemaker functions. The power source 214 may include one or more energy storage devices, such as one or more rechargeable or non-rechargeable batteries. For clarity, connections between power supply 214 and other pacemaker circuits and components are not shown in fig. 3.

Telemetry circuitry 208 includes a transceiver 250 for transmitting and receiving data via a Radio Frequency (RF) communication link and an antenna 252. As described above, telemetry circuitry 208 may be capable of two-way communication with external device 20 (FIG. 1). The accelerometer (or other motion sensor) signals and the EGM signals, and/or data derived therefrom, may be transmitted by telemetry circuitry 208 to external device 20. Programmable control parameters and algorithms for performing atrial event detection and ventricular pacing control may be received by telemetry circuit 208 and stored in memory 210 for access by control circuit 206.

FIG. 4 is a schematic diagram of atrial event monitor circuitry 240 according to an example. In this example, the motion sensor 212 is referred to as an accelerometer. Atrial event detector circuit 240 may include a pre-filter 260 that receives raw accelerometer signals from at least one axis of accelerometer 212. The pre-filter 260 may be a high pass or band pass filter that removes DC offset and high frequency noise and may include an amplifier for amplifying the signal passed to the rectifier 262. Rectifier 262 includes a rectifier and may include additional amplification and filtering circuitry for passing the amplified, filtered and rectified signal to blanking circuitry 264.

The blanking circuit 264 may blank or smooth the accelerometer signal during a ventricular blanking period that is set based on input received from the sensing circuit 204. For example, blanking circuit 264 may receive the R-wave sensed event signal from R-wave detector 224 and set a blanking period applied to the accelerometer signal that encompasses a time window corresponding to at least the QRS stage of the cardiac signal or encompasses an R-T interval or a Q-T interval. Comparator 268 receives the rectified accelerometer signal from blanking circuit 264 and compares the accelerometer signal to an atrial event detection threshold. During the ventricular blanking period, blanking circuit 264 may not pass the accelerometer signal to comparator 268 or pass a zero signal. Blanking may be performed by suppressing power from the amplifier and/or other components of atrial event detector circuit 240 for conserving power and reducing the likelihood of erroneously detecting a ventricular event as an atrial mechanical contraction. In other examples, the accelerometer signal may be smoothed or averaged during the ventricular blanking period such that the rectified accelerometer signal is maintained at an amplitude less than the atrial event detection threshold during the ventricular blanking period.

In other examples, atrial event detector 240 may include a morphology analyzer 266, which morphology analyzer 266 may be a microprocessor-based circuit for analyzing the morphology of the digitized cardiac electrical signal received from ADC 226. The morphology analyzer 266 may be configured to perform waveform analysis for identifying R-waves and/or T-waves from the cardiac electrical signal for setting a ventricular blanking period by the blanking circuit 264. For example, blanking circuit 264 may receive a timing signal from morphology analyzer 266 upon detection of an R-wave (associated with ventricular depolarization and contraction) and begin a predefined ventricular blanking period expected to contain T-waves (associated with ventricular repolarization and diastole). The ventricular blanking period may be set to a percentage of the RR interval, which is the time interval between successive sensed R-waves, since the QT interval generally shortens as the heart rate increases. In one example, the ventricular blanking period may be set to at least 200ms and up to 650ms, or between 20% and 80% of the RR intervals. In other examples, the ventricular blanking period may be set based on a history of RR intervals between consecutive sensed R-waves or a history of AA intervals between consecutive detected atrial events. For example, the ventricular blanking period may be the most recent RR or AA interval (or an average of a predetermined number of most recent intervals) less than a predetermined time interval, e.g., less than 100ms, less than 125ms, less than 150 milliseconds, or other predetermined interval.

In other examples, the blanking circuit 264 may receive the R-wave sensed event signal from the R-wave detector 224 or from the morphology analyzer 266 to begin a ventricular blanking period based on the identified R-wave. The blanking circuit 264 may also receive a T-wave sensed event signal from the morphology analyzer 266 for terminating the ventricular blanking period based on the T-wave identified after the R-wave. The ventricular blanking period may begin before the R-wave sensed event signal, and thus the blanking circuit 264 may include a delay applied to the accelerometer signal passed to the comparator 268. The AV pacing interval started in response to a detected atrial mechanical event may be adjusted for any delay introduced by blanking circuit 264 (or other components of atrial event detector circuit 240).

The blanking of the accelerometer signal may begin at a ventricular sense event or a ventricular pace event and may continue for a predetermined blanking period after the amplitude of the rectified near-field cardiac electrical signal acquired by the sensing circuit 204 remains below a particular threshold (e.g., 0.1-0.5mV) for a particular number of consecutive samples (e.g., 10 samples at a sampling rate of 256 Hz). In some examples, the blanking period may be 200 to 250 ms. The blanking period may be modified based on the intrinsic heart rate or the ventricular pacing rate, with shorter and shorter intervals as the heart rate increases. In another example, the morphology analyzer 266 may determine the slope of the cardiac electrical signal for use in determining the end of the blanking period. The isoelectric T-P segment on the near-field ventricular cardiac signal may be detected based on a low slope criterion applied by the morphology analyzer 266. Blanking circuit 264 may terminate the ventricular blanking period applied to the accelerometer signal such that comparator 268 is enabled to detect atrial mechanical events occurring after electrical depolarization of the atrium (corresponding to the P-wave).

In yet other examples, blanking circuit 264 may pass the rectified accelerometer signal to comparator 268, and comparator 268 may detect a ventricular event threshold crossing of the rectified accelerometer signal. The ventricular event threshold may be set to a high value in order to detect an accelerometer signal associated with ventricular contraction that will be much greater than an accelerometer signal associated with atrial kick. The comparator 268 may provide a feedback signal 280, the feedback signal 280 indicating when the rectified accelerometer signal crosses the ventricular event threshold. The blanking circuit 264 may apply a ventricular blanking period to the accelerometer signal in response to receiving the ventricular event feedback signal 280. If atrial event detection criteria are met during the blanking period, e.g., if the accelerometer signal crosses a threshold amplitude and/or slope, atrial event detector circuit 240 ignores the event and does not generate A-output signal 284. Instead of (or in addition to) setting the blanking period, the atrial event sensing window may be set based on detecting a ventricular event from the accelerometer signal.

In some examples, the vibration signal of the accelerometer signal may be analyzed for determining a start and an end of a ventricular ejection phase for providing a start and an end of a ventricular blanking period applied to the accelerometer signal for detecting atrial events outside of the ventricular blanking period. Blood flow during ventricular ejection may be detected by comparator 268, and the end of the ejection phase and the beginning of the electrical repolarization phase may be detected when the amplitude, frequency, slope or other characteristic of the accelerometer signal falls below a threshold. Detecting the beginning and end of the ventricular ejection phase based on the vibration of the accelerometer can be used to set the ventricular blanking period applied to the accelerometer signal for two purposes: ignoring atrial event detections that may occur during this time interval, and controlling the duration of a ventricular sense refractory period applied to the cardiac signal after the R-wave sensed event signal for reducing the likelihood of R-wave oversensing and maximizing the time that the inductive circuit 204 is not blanked.

Blanking circuit 264 may pass the rectified accelerometer signal to comparator 268, which comparator 268 compares the received signal to an atrial event detection threshold. Blanking circuit 264 is optional and may be excluded in some examples. The rectified accelerometer signal may be continuously received by the comparator 268 and/or the atrial event discriminator 272, and atrial mechanical event detection criteria may be applied to the accelerometer signal by the comparator 268 and/or the atrial event discriminator 272 for detecting and discriminating atrial mechanical events from ventricular mechanical events.

In some examples, the absolute amplitude of the accelerometer signal may be on the order of 0.1g, and the amplitude threshold for detecting atrial mechanical events may be set below the maximum expected absolute amplitude. If the rectified accelerometer signal meets or crosses the atrial event detection threshold outside of the ventricular blanking period (or within the atrial event window), the atrial event signal 282 may be passed to the atrial event discriminator 272, which may apply other criteria before confirming atrial event detection and passing the atrial mechanical event detection output signal (A-out) 284 to the pacing timing circuit 242.

In other examples, other aspects of the accelerometer signal may be determined and compared to atrial event detection criteria. For example, atrial event detector circuit 240 may include a differentiator or other signal processing circuit for processing the digital accelerometer signal for determining the derivative of the accelerometer signal. The maximum derivative or slope of the accelerometer signal may be compared to a slope threshold for detecting atrial events. In other examples, the detector circuit 240 may include an integrator to integrate the rectified accelerometer signal and detect an atrial event when the integrator output reaches a threshold outside of the ventricular blanking period. In various examples, the amplitude, slope, area, frequency, or other signal characteristic of the vibration or signal spike may be determined and compared to atrial event detection criteria for detecting atrial mechanical events corresponding to atrial contractions and active filling phases of the ventricles.

Other atrial event detection criteria may be applied to one or more points in time determined from the accelerometer signal, such as the start time, duration, and end time of a vibration or signal spike associated with blood flow during active ventricular filling (atrial contraction), and these other atrial event detection criteria may require reaching a threshold requirement that indicates an expected vibration intensity (e.g., amplitude, frequency, or slope of the accelerometer signal) during all or at least a portion of the duration of the active ventricular filling phase through the tricuspid valve. The point in time for atrial event detection to begin an AV interval may be based on one or more fiducial points or characteristics of the accelerometer signal.

In some examples, atrial event discriminator 272 may receive the atrial sense signal from comparator 268 and determine, in response to the sense signal, whether a peak-to-peak amplitude difference of the cardiac signal does not exceed the maximum P-wave threshold for a predetermined time interval prior to the atrial sense signal received from comparator 268. Verification of a relatively low amplitude cardiac electrical signal during a time interval prior to accelerometer-based atrial event detection may be used to confirm that an accelerometer signal threshold crossing (or other signal characteristic that meets atrial event detection criteria) is not caused by a ventricular event. The maximum P-wave amplitude threshold may be set to be greater than the expected maximum peak-to-peak amplitude of the P-waves present in the cardiac electrical signal but less than the expected amplitude of the T-waves or R-waves. The maximum P-wave amplitude threshold may be between 0.1mV and 1mV (including 0.1mV and 1 mV). In one example, the maximum P-wave amplitude is no greater than 1.5 mV. If the peak-to-peak amplitude detector 270 detects a peak-to-peak amplitude greater than a threshold value during a predetermined time interval prior to the atrial sense signal from the comparator 268 (e.g., within 80ms prior to the atrial sense signal), the peak-to-peak detector 270 may pass a signal to the atrial event discriminator 272 that causes the atrial event discriminator 272 to suppress detecting atrial mechanical events. The presence of a high peak-to-peak amplitude difference in the cardiac electrical signal may indicate that a ventricular event has occurred that results in an increase in the amplitude, slope, vibration frequency, or other characteristic of the accelerometer signal used to detect atrial events.

If atrial event discriminator 272 does not receive a signal from peak-to-peak detector 270 indicating a peak-to-peak amplitude difference that exceeds the maximum P-wave threshold during a previous time interval, atrial event discriminator 272 generates an atrial mechanical event output signal (a-output) in response to the accelerometer signal crossing the atrial event detection threshold (and/or other accelerometer signal characteristic(s) that meet atrial event detection criteria). The a-output signal from the atrial event detector signal 240 is delivered to the pace timing circuit 242 and causes the pace timing circuit 242 to start the AV pacing interval. At the expiration of the AV pacing interval, the pulse generator 202 delivers a pacing pulse to the RV that is synchronized with atrial mechanical events without an intervening R-wave sensed event signal from the sensing circuitry 204.

Fig. 5 is a flowchart 300 of a method for delivering atrial synchronous ventricular pacing by pacemaker 14 according to one example. Although fig. 5 is described in connection with an accelerometer, other motion sensor signals may be substituted. At block 302, atrial event detector circuit 240 acquires the rectified accelerometer signal. At block 304, atrial event detection criteria are set, which may be one or more user programmable values and may be based on individual patient signals or empirical clinical data. In one example, at least one detection amplitude threshold is set. In other examples, at least one detection slope threshold is set. The detection criteria may relate to one or more accelerometer signal characteristics, such as, without limitation intended, the peak amplitude of the signal spike(s) associated with blood flow into the ventricle during active filling phases, the frequency of the spike, the slope and/or region of the one or more spikes during active filling phases (outside of ventricular blanking).

In some examples, the atrial event detection criteria are automatically set by the control circuitry 206. For example, the control circuit 206 may buffer the rectified accelerometer signal in the memory 210 and determine an active filling phase time window based on the R-wave sensed event signal from the sensing circuit 204. For example, the active filling phase window may be a 150ms window that ends at or just before (e.g., 10ms before) the event signal sensed by the R-wave. The maximum amplitude of the rectified accelerometer signal may be determined during the window. An atrial event detection amplitude threshold for detecting subsequent atrial mechanical events may be automatically set to a predetermined percentage (e.g., 75%, 50%, or other percentage) of the average maximum amplitude based on the maximum amplitude measurement over one or more cardiac cycles. In other examples, an average maximum slope or other accelerometer signal characteristic may be determined during the active filling phase window, and an atrial event detection threshold may be set based on the determined characteristic.

If the sensing circuit 204 does not sense an intrinsic ventricular sensed event, e.g., because the patient has complete AV block, the control circuit 206 may inhibit ventricular pacing for a particular period of time (e.g., one to three seconds) to measure the maximum amplitude, slope, and/or other characteristics of the accelerometer signal during the no-pacing interval. For example, the control circuitry 206 may control the pulse generator 202 to deliver asynchronous ventricular pacing at a base rate of 40 to 60 bpm. The control circuit 206 may control the pulse generator 202 to skip a pacing pulse to provide a two to three second asystole interval. In the absence of any ventricular event (sensing or pacing), the maximum amplitude, maximum slope, maximum signal spike region, etc. during this period of ventricular asystole are expected to represent atrial mechanical events. The control circuit 206 may analyze the middle portion of the asystole period when a suppression pacing pulse is scheduled to occur to detect an accelerometer signal spike expected to be associated with atrial activity in the absence of ventricular activity. The atrial event detection threshold for subsequent detection of atrial mechanical events during ventricular pacing may be set to a predetermined percentage of the average maximum amplitude, slope, area, for example, determined in the absence of ventricular pacing.

At block 306, ventricular event detection criteria may be set for detecting ventricular mechanical events based on the accelerometer signal. The ventricular event threshold may be set well above the atrial event threshold for detecting blood flow acceleration due to ventricular contraction or vibration due to the flow of the anchors 165. The ventricular event threshold may be determined from the accelerometer signal after ventricular electrical events of the R-wave are observed (as the cardiac electrical signal received by the sensing circuit 204). In some examples, the starting point, duration, and ending point of a ventricular ejection phase are determined from flow-induced vibration signals in the accelerometer signal for setting a ventricular blanking period or an atrial event sensing window.

At block 308, atrial event detector circuit 240 determines whether a ventricular event has been detected. Ventricular events may be detected based on the R-wave sense event signal received from the sensing circuit 204, based on morphological analysis of the cardiac electrical signal by the morphology analyzer 266, or based on the rectified accelerometer signal crossing a ventricular event amplitude threshold or other ventricular event detection threshold established at block 306. If a ventricular event is detected, atrial event detector circuit 240 sets a ventricular blanking period during which the accelerometer signal is blanked or smoothed to exclude any atrial event threshold crossing, and/or any atrial event threshold crossing that does occur is ignored, at block 310. In some examples, the ventricular blanking period may be a variable length period controlled by monitoring flow-induced vibration spikes of the accelerometer signal following the R-wave sense event signal. When the flow-induced vibration spike decreases below the ventricular event detection criteria, the end of the ventricular ejection phase is detected and the ventricular blanking period may be terminated.

The atrial event detector circuit 240 may set a P-wave window at block 314 if an atrial event detection threshold crossing is detected outside of the ventricular blanking period set at block 310, or if a ventricular blanking period has not been set ("no" branch of block 308) but a threshold crossing is detected ("yes" branch of block 312), at block 312. The P-wave window set at block 314 may be set to confirm that the cardiac electrical signal during the time window prior to the atrial event threshold crossing of the accelerometer signal does not exceed the peak amplitude threshold (or peak-to-peak difference threshold). If the maximum peak amplitude (or peak-to-peak difference) of the cardiac signal during the P-wave window exceeds a peak amplitude threshold, suggesting the presence of an R-wave or T-wave during the P-wave window, the atrial event detection threshold crossing of the accelerometer signal may be caused by a ventricular event rather than an atrial event.

At block 316, the maximum peak amplitude of the rectified cardiac signal or the maximum peak-to-peak difference of the unrectified cardiac signal is determined by the peak detector 270 of the atrial event detector circuit 240. Atrial event discriminator 272 determines whether the peak amplitude threshold is exceeded during the P-wave window preceding atrial event signal 282 received from comparator 268. If the peak amplitude threshold is exceeded ("no" branch of block 316), no atrial mechanical event is detected. Accelerometer signal threshold crossing may be caused by a ventricular event. The process returns to block 308 to monitor for the next accelerometer signal threshold crossing.

If the peak amplitude threshold has not been exceeded ("yes" branch of block 316), an atrial mechanical event is detected at block 318. The atrial event detection signal is delivered to pacing timing circuit 242. In response to atrial mechanical event detection, the pacing timing circuit 242 schedules a ventricular pacing pulse by setting an AV pacing interval at block 320.

The scheduled ventricular pacing pulse may be cancelled if an R-wave is sensed by the sensing circuitry 204 during the AV pacing interval. At block 310, a ventricular blanking period may be set in response to the sensed R-wave, and the process repeated. If the AV pacing interval expires, the scheduled ventricular pacing pulse is delivered by the pulse generator 202 at block 324. A ventricular blanking period may be set in response to delivering a ventricular pacing pulse at block 310, and the process continues for detecting a next atrial mechanical event based on the blood flow induced accelerometer signal.

Fig. 6 is a diagram of accelerometer signal 402, EGM signal 420, and timing diagram 440 representing operations performed by pacemaker 14. Accelerometer signal 402 is shown as a rectified signal generated by atrial event detector circuit 240 from the raw signal received from accelerometer 212. The EGM signal 420 is generated by the sensor circuit 204 by filtering and amplifying the raw cardiac signal received via the

electrodes

162 and 164.

The accelerometer signal 402 includes high amplitude peaks 403 caused by ventricular contractions and corresponding in time to the R-wave 422 of the EGM signal 420. The accelerometer signal 402 further includes a peak 405 of relatively low amplitude, which peak 405 corresponds in time to a T-wave 424 of the EGM signal 420. Both of these

events

403 and 405 are ventricular events. The accelerometer signal 402 further includes a peak 407 that corresponds in time to the P-wave 426 of the EGM signal 420, and is therefore an atrial event. While the high amplitude peak 403 corresponding to ventricular contraction is easily distinguished from the peak 407 corresponding to atrial contraction (due to increased blood flow through the tricuspid valve during active ventricular filling), the peak 405 corresponding to the T-wave is associated with ventricular relaxation at the end of ventricular ejection and is similar in amplitude to the atrial event peak 407.

EGM signals 420 include R-waves 422 that accompany ventricular depolarization, T-waves 424 that accompany ventricular repolarization, and P-waves 426 that accompany atrial depolarization. Atrial event detector circuit 240 of fig. 4 may communicate (on timeline 440) ventricular event signals 442 associated with ventricular depolarization or ventricular contraction to blanking circuit 264, blanking circuit 264 setting a ventricular blanking period 450 in response to each identified ventricular event. The ventricular event signal 442 may be generated in response to the accelerometer signal 402 crossing the ventricular event threshold 414, the R-wave sensed event signal generated by the sensing circuit 204 in response to the EGM signal 420 crossing the R-wave sensing threshold, the R-wave 422 detected by the morphology analyzer 266 in response to the delivery of ventricular pacing pulses by the pulse generator 202, or any combination thereof. During blanking period 450, the crossing of atrial event detection amplitude threshold 404 may be ignored by atrial event detector circuit 240, or accelerometer signal 402 may be blanked or smoothed, such that an atrial event detection threshold crossing does not occur. The atrial event detector circuit 240 does not generate an atrial event detection signal during the ventricular blanking period 450.

Atrial event detector circuit 240 detects atrial event detection

amplitude threshold crossings

406 and 408 via accelerometer signal 402. In response to

threshold crossings

406 and 408, atrial event detector circuit 240 sets P-

wave windows

444 and 446, respectively, to verify that the peak-to-peak amplitude difference of EGM signal 420 does not exceed peak amplitude threshold 430 during P-

wave windows

444 and 446, respectively, prior to atrial

event threshold crossings

406 and 408. Since the maximum peak-to-peak amplitude difference does not exceed the peak amplitude threshold 430 during the P-

wave sensing windows

444 and 446, both atrial event detection

amplitude threshold crossings

406 and 408 result in atrial mechanical event detection signals 460 and 462, respectively, generated by the atrial event detector circuit 240. The atrial mechanical event detection signals 460 and 462 are delivered to the pacing timing circuit 242, and the pacing timing circuit 242 starts

AV pacing intervals

480 and 482 in response to each of the atrial event detection signals 460 and 462, respectively.

The R-wave 422 is sensed by the sensing circuitry 204 during the AV pacing interval 480. The pace timing circuit 242 cancels the scheduled ventricular pacing pulse and waits for the next atrial mechanical event detection signal 462. The next AV pacing interval 482 expires without an R-wave sensed event. The pulse generator 202 delivers a ventricular pacing pulse 443 at the expiration of the AV pacing interval 482. In response to the ventricular pacing pulse 443, the next ventricular blanking period 450 begins.

The next atrial event threshold crossing 410 occurring outside of ventricular blanking period 450 causes atrial event detector circuit 240 to set P-wave window 470 to check for the peak-to-peak amplitude difference of EGM signal 420 during the P-wave window prior to threshold crossing 410. Since in this case, the peak-to-peak amplitude difference 432 exceeds the peak amplitude threshold 430, the atrial event threshold crossing 410 is not detected as an atrial mechanical event even if the threshold crossing occurs outside of the ventricular blanking period 450. The atrial event threshold crossing 410 corresponds in time to the T-wave 424 and is not correctly confirmed as an atrial event by the atrial event detector circuit 240 based on EGM amplitude analysis.

However, the next atrial event threshold crossing 412 does correspond in time to the P-wave 426 and is detected as an atrial mechanical event after analyzing the EGM signal amplitude during the P-wave window 472 to verify that the peak amplitude threshold 430 has not been exceeded. The pace timing circuit 242 sets the AV pacing interval 484 in response to the atrial event detection signal 464 from the atrial event detector circuit 240.

It may be appreciated that in any of these examples, the control circuitry 206 may provide backup, asynchronous ventricular pacing at a predetermined backup pacing interval in the event that an atrial mechanical event is not detected. The backup pacing interval may be set to a predetermined base rate interval, e.g., 1 second or longer. Asynchronous backup pacing is provided at a base rate interval after a previous ventricular pacing pulse or sensed R-wave to prevent asystole when an atrial mechanical event is not detected and the AV interval is not set.

Thus, various examples of intracardiac pacemakers configured to deliver atrial-synchronized ventricular pacing have been described in accordance with illustrative embodiments. In other examples, the various methods described herein may include steps that are performed in a different order or combination than the illustrative examples shown and described herein. In addition, other circuitry for implementing the techniques disclosed herein may be apparent to one of ordinary skill in the art; the specific examples described herein are illustrative in nature and not restrictive. It is understood that various modifications may be made to the reference examples without departing from the scope of the disclosure and the following claims.

Claims

1. An intracardiac ventricular pacemaker comprising:

a housing;

a pulse generator within the housing and configured to generate and deliver pacing pulses to a ventricle of a heart of a patient via electrodes coupled to the pacemaker;

a motion sensor within the housing and configured to generate a motion sensor signal; and

a control circuit within the housing and including atrial event detector circuitry coupled to the motion sensor;

a pacing timing circuit within the housing and coupled to the pulse generator; and

a flow perturbation on the housing, wherein the flow perturbation is configured to: generating vibrations in response to movement of blood within the ventricle during atrial contraction and transmitting the vibrations to the motion sensor within the housing, and

wherein the atrial event detector circuit is configured to detect an atrial mechanical event from the motion sensor signal of the motion sensor in response to the vibration and to deliver an atrial event signal to the pacing timing circuit in response to detecting the atrial mechanical event,

the pacing timing circuit is configured to schedule a pacing pulse by starting a pacing interval in response to receiving the atrial event signal, and

the pulse generator is configured to deliver the scheduled pacing pulse to a ventricle of the patient's heart in response to expiration of the pacing interval.

2. The pacemaker of claim 1, wherein the atrial event detector circuit is configured to detect the atrial mechanical event by detecting a threshold crossing of the motion sensor signal.

3. The pacemaker of claim 1, wherein the atrial event detector circuit is further configured to:

detecting a ventricular mechanical event from the motion sensor signal;

setting a ventricular blanking period in response to the ventricular mechanical event detection; and

detecting the atrial mechanical event in response to the motion sensor signal satisfying atrial event detection criteria outside of the ventricular blanking period.

4. The pacemaker of claim 1, further comprising a sensing circuit configured to receive a cardiac electrical signal via the electrode, sense a ventricular electrical event from the cardiac electrical signal, and generate a ventricular sensed event signal in response to sensing the ventricular electrical event;

the atrial event sensor circuit is configured to receive the ventricular sense event signal, set a ventricular blanking period in response to the ventricular sense event signal, and detect the atrial mechanical event in response to the motion sensor signal satisfying an atrial event detection criterion outside of the ventricular blanking period.

5. The pacemaker of claim 1, further comprising a sensing circuit configured to receive cardiac electrical signals via the electrodes,

the atrial event detector circuit is further configured to:

setting a P-wave window in response to detecting the atrial mechanical event;

comparing the cardiac electrical signals received during the P-wave window to atrial event confirmation criteria; and

delivering the atrial event signal to the pacing timing circuit in response to the atrial event confirmation criteria being met.

6. The pacemaker of claim 5, wherein the atrial event detector circuit is further configured to:

comparing the cardiac electrical signal to atrial event confirmation criteria by determining a peak-to-peak amplitude difference of the cardiac electrical signal during the P-wave window;

comparing the peak-to-peak amplitude difference to a peak amplitude threshold; and

determining that the atrial event confirmation criteria are satisfied in response to the peak-to-peak amplitude difference being less than the peak amplitude threshold.

7. The pacemaker of any one of claims 1-6, wherein the atrial event detector circuit is configured to detect the atrial mechanical event by determining at least one of a first derivative, a peak slope, an area, a peak amplitude, and a frequency from the motion sensor signal.

8. The pacemaker of any one of claims 1-6, wherein the housing comprises:

a housing proximal end and a housing distal end; and

a lateral groove that causes the vibration of the housing when the pacemaker is subjected to blood flowing into the ventricle during atrial contraction.

9. The pacemaker of any one of claims 1-6, wherein the housing is configured to carry the electrode.

10. An intracardiac ventricular pacemaker comprising:

a pulse generator configured to generate and deliver pacing pulses to a ventricle of a patient's heart via electrodes coupled to the pacemaker;

a motion sensor configured to generate a motion sensor signal related to the motion of blood within the ventricle; and

a control circuit comprising an atrial event detector circuit coupled to the motion sensor, and a pacing timing circuit coupled to the pulse generator;

a housing enclosing at least the pulse generator and the control circuit and having a housing proximal end and a housing distal end;

a fixation member coupled to the housing distal end for anchoring the housing at an implantation site; and

a flow-perturbing fixture having a first end coupled to the housing proximal end and a second end extending away from the housing, the second end configured to vibrate when the fixture is subjected to blood flow into the ventricle during atrial contraction, the first end configured to transmit the vibrations to the motion sensor,

wherein the atrial event detector circuit is configured to detect an atrial mechanical event from the motion sensor signal and to deliver an atrial event signal to the pace timing circuit in response to detecting the atrial mechanical event,

the pacing timing circuit is configured to schedule the pacing pulse by starting a pacing interval in response to receiving the atrial event signal,

11. The pacemaker as described in claim 10, wherein said anchor comprises a first stiffness along a first axis of said anchor and a second stiffness along a second axis of said anchor, said second stiffness being less than said first stiffness,

the motion sensor comprises at least one motion axis;

the second axis is aligned with at least one axis of the motion sensor.

12. The pacemaker as described in claim 11, wherein said fixation member has a first width along said first axis and a second width along said second axis, said second width being less than said first width.

13. The pacemaker as described in any one of claims 10-12, wherein said anchor is radially asymmetric with respect to said housing.

14. The pacemaker as described in any one of claims 10-12, wherein said fixation member is adjustable by at least one of rotation and bending.

15. The pacemaker of any one of claims 10-12,

the housing includes lateral grooves that cause vibration of the housing when the pacemaker is subjected to blood flow into the ventricle during atrial systole.

16. The pacemaker of any one of claims 10-12, wherein the housing is configured to carry the electrode.